Tubular fuel pencil having a thin flexible can for nuclear reactors



March 7, 1967 ALFILLE ETAL 3,308,033

TUBULAR FUEL PENCIL HAVING A THIN FLEXIBLE CAN FOR NUCLEAR REACTORS Filed July 20, 1965 SWW/Mom,

Lucier Hifi/le eawC/aude Charnan/ mam.

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United States Patent 3,308,033 TUBULAR FUEL PENCIL HAVING A THIN FLEXI- ELE CAN FUR NUCLEAR REACTGRS Lucien Alflle, Paris, France, and Jean Claude Charrault, Rasee-Upanne, and Francois Lafontaine, Ispra, Italy, assignors to European Atomic Energy Community, Brussels, Belgium Filed .Iuly 20, 1965, Ser. No. 473,420 Claims priority, application Belgium, Aug. 14, 1964, 1,852/ 64 11 Claims. (Cl. 176-72) The invention relates to a tubular fuel pencil for nuclear reactors, including for example pressure fluid cooled power reactors, the pencil mainly comprising a cylindrical outer can and a cylindrical inner can which co-operate to receive in the annular space between the cans, .a tube or stack of rings of fissile material and two plugs for connecting and closing the ends of the said space.

Pencils of this form are desirable in the case of fuels of high specic power, the fuel being in an annular shape with the advantage that the outside and inside surfaces of the pencil may be cooled, the idea being to increase greatly the heat-exchange surfaces in Contact With the heat vehicle fluid for a given fuel cross-section. It is also known that, except for plastic or flexible cans which are engaged with the fuel by the action of the pressursed heat vehicle fluid, the fuel being metallic such as uranium or uranium-based alloys, the use of pencils of the kind described is in general very limited because of the dilculties in providing adequate thermomechanical stability of the inner can. One reason for this is that, in the common case where the coefficient of expansion of the cans is greater than the coefllcient of expansion of the fuel, the inner can tends to expand, bears against the inside surface of the tubular fuel as it does so and is therefore cornpressed, with the result that in some temperature conditions the inner can is made unstable by local buckling which leads to the formation of folds or corrugations which in turn quickly lead to leaks. This kind of thermomechanical instability of the inner can is found with rigid cans not connected to the fuel, with cans connected to the fuel, for instance, by a diffusion barrier-in which case the connection first shears, whereafter folds or corrugations are formed-and with pencils having a rigid can not connected to the fuel and having a coeflicient of expansion less than the coefficient of expansion of the fuel. The reason for the latter case is that when the fuel diseagages from the can because of expansion, the temperature of the inside surface of the tubular fuel rises, leading to further disengagement lof the fuel from the can and increased thermal and mechanical `stressing of the canning material and of the fuel.

lt is also known to use in cylindrical fuel pencils a very thin can of a thickness of 0.1 mm. or less, the can being made flexible by means of corrugations permitting radial and axial deformation, the can being placed around the fuelwhich can take the form of a rod -or of a stack of pastilles-with the interposition of a material which is plastic at the working temperatures of the reactor. ln one such fuel element, disclosed by the applicants US. patent application Serial No. 288,081, filed on lune 17, 1963, under the title of Fuel Rods for Nuclear Reactors, now Patent No. 3,274,069, issued September 20, 1966, the can, which is too thin to be inherently rigid, can be produced in large diameters, stability being provided by deformation-permitting folds or corrugations whose convexity faces outwards and which are provided on that part of the can which is between the terminal closure-receiving zones (the folds or corrugations comprising longitudinal folds or corrugations per- 3,308,033 Patented Mar. 7, 1967 ice mitting radial deformation and, at at least one end of said part, peripheral or helical folds or corrugations which permit axial deformation and which are in extension of the longitudinal folds or corrugation), the folds and the clearance between the can and the fuel being filled by the material which is plastic when hot and which is preferably magnesium. In this system the can can withstand very high external pressures without suffering severe stressing by engaging with the hydrostatically stressed plastic cushion or the like, and the can can withstand forces caused by expansion of the fuel and of the plastic cushion and changes inthe dimensions of the fuel duringradiation, for all such changes, being transmitted to the corrugations by hydrostatic deformation, do not overstress the can material.

It is also known that these very thin-canned cylindrical fuel pencils have very reduced transverse rigidity, more particularly in the case of a stack of fuel pastilles, and must therefore either have a stiffening structure around them or be placed in a channel which imposes a definite geometry near them. It is also desirable that they have longitudinal centering folds or corrugations strengthened by a stiffening or strengthening wire, to enable the pencils to remain in a predetermined axial position in the vein of cooling fluid flowing through the structural channel around the pencil. Pencils of this construction form the subject matter of the applicants U.S. patent application Serial No. 472,972, filed July 19, 1965, entitled Fuel Pencils Having a Thin Flexible Can for Nuclear Reactors.

This invention relates to a tubular fuel pencil designed to have improved thermal and mechanical characteristics in order to reduce the thermomechanical instability associated with the inner can of the known constructions.

In its broad form the invention provides a tubular fuel pencil for a nuclear reactor comprising tubular inner and outer cans defining between them an annular space containing a tube or a stack of rings of nuclear fuel and means for closing the ends of the annular space characterized in that the inner can has inwardly projecting longitudinal corrugations and that there is a rigid cylindrical member within the inner can against which the corrugations may bear to support the can.

More specifically the invention provides a fuel pencil of the kind described which comprises thin flexible cans separated from the fuel by a material which is plastic when hot and which serves to compensate for differences between the heat expansion of the fuel and the heat expansion of the inner can and to reduce the risk of local buckling or thermal instability, a central stiflening element being provided for improved transverse stiffness.

In 'one form of the invention, the pencil comprises thin (eg. 0,1 mm. or less in thickness) flexible cans which are preferably made of stainless steel and which have, in the part between the terminal plug-receiving zones, longitudinal folds or .corrugations permitting radial deformation and, at least one end of said part, peripheral or helical folds or corrugations permitting axial deformation, the cans having equidistant longitudinal folds or corrugations whose fold or co-rrugation height is greater than the fold or corrugation height of the corrugations or folds permitting axial deformation, the equidistant longitudinal folds or corrugations being internally strengthened by a strengthening or stiffening wire, the cans receiving between them the fuel with the interposition of a viscous layer of a material which is plastic at the working ternperatures of 4the pencil in the reactor, there being introduced into the hollow interior of the pencil a rigid. cylindrical member against whose surface the wire-strengthened longitudinal folds or corrugations of the inner can bear and whose ends are rigidly secured to the pencil ends.

Other features and details of the invention will be disclosed by the followin-g description of specific examples, reference being made to the accompanying drawings wherein:

FIGURE 1 is a cross-section through one example of a tubular fuel pencil according to the invention;

FIGURE 2 is a section on the line A-A of the pencil shown in FIGURE 1, and

FIGURE 3 is a partial longitudinal-section of an alternative form of the pencil shown in FIGURES 1 and 2.

The drawin-gs show for each example, a pencil inner can 1 and outer can 2 (also called sheaths, clads, or jackets), fuel rings 3, which can be of metallic uranium or of a uranium-based allow or of ceramics, such as uranium carbide or uranium oxide, stacked one above another in the space between the inner can and the outer can; annular plugs 4 which connect and close the annular space between the cans, have raised edges 5 and 6 and annular projections 7 for supporting several pencils of a stack; and two refractory ceramic rings 8 placed between the fuel and the plugs to heat insulate the same from the fuel end faces.

The cans 1 and 2, are very thin i.e. have a thickness of 0.1 mm. or less-eg., from 0.05 to 0.06 mm., are preferably of stainless steel and have longitudinal folds or corrugations 9 in the part between the terminal plugreceiving zones. They also have, in the terminal zones of said part, peripheral or helical folds or corrugations 10 in extension of the longitudinal corrugations 9. The cans are separated from the fuel by viscous layers 11 formed by a material, such as magnesium, which is plastic at the working temperatures of the pencil in the reactor. The annular gaps between the can 1 and the inside surface of the rings 3 and between the outside surface of the rings 3 and the can 2 are completely filled by the material which is plastic when hot so that, since the tubular pencil experi-ences a hydrostatic pressure, for instance, from the cooling fluid, the viscous -layer or cushion 11 is compressed hydrostatically through the agency of the flexible cans. The corrugations therefore have internal support from the triaxially stressed plastic metal and do not become fatigued because of geometric variations in the pencil elements due to different heat operating conditions in the reactor. Consequently, the inner can 1 experiencing the pressure of the cooling fluid is forced against the inside surface of the fuel 3 with the interposition of the plastic cushion 11, and the corrugations 9 and 10 reduce thermomechanical instability by compensating for ditferences between the expansion of the fuel and the expansion of the can by exing, thus reducing the risk of local buckling or of heat instability.

For transverse stiifening of the pencil, a rigid cylindrical member 12 is placed in the pencil coaxially thereof and is borne by the plug rings 7, and stilfening or strengthening wires 13 are placed in the longitudinal oorrugations 9. The cylindrical member 12, which is preferably made of nuclear graphite, is rigidly connected to the cans by means, for instance, of spring steel wires 14 which are disposed in a cross and secured in apertures 15 in the rings 7. One of the wires 14, which are placed one above another, is embedded in a diametric groove 16 in the member 12. The wires 13 inside the corrugations 9, which bear against the fuel 3, help the corrugations 9 of the inner can to center the pencil on the member 12 and help the corrugations 9 of the outer can which bear against the surface of the cooling channel 17 in which the pencil is used; the channel 17 can be either an individual forcing or pressure tube of the kind disclosed by the applicants U.S. patent application Serial No. 385,194, led July 27, 1964, and now abandoned in favor of continuation application Serial No. 571,945 filed August 11, 1966 for Assembly of Fuel Elements for Nuclear Reactors, or the dismantleable structure for a bunch disclosed by the applicants U.S. patent application Serial 4 No. 263,351, filed March 6, 1963, entitled Nuclear fuel elements and nowV abandoned.

The longitudinal corrugations 9 are of a height which is greater than the height of the corrugations 10, to provide an adequate cooling fluid flow cross-section at the end zones of the pencil, and which is such as to ensure the effective cross-sections required for the cooling -uid veins 18 and 19 flowing between the surface of the can 1 and the member 12 and between the surface of the can 2 and the channel 17.

The lcorrugations 9` are connected to the corrugations- 10, if the latter are helical, by corrugations of progres-- sively varying height. To increase the heat-exchange area of the cans 1 and 2, the same are formed, on those parts of their surfaces which are between the corrugations 9i permitting radial deformation, with small cooling folds or corrugations 20 which extend parallel with the corrugations 9 and whose length is not 'greater than the length of the corrugations 9.

Preferably, the plugs 4 take the form of rings which are made of stainless steel or of the Isame material as is used for the cans, and the raised edges 5 and 6 of ther plugs are simultaneously respectively welded to the rings- 21 and 22 either by angon arc welding or by electron bombardment, to form a closure similar to the simple pencil closure disclosed by the applicants U.S. patent. application Serial No. 433,545, led February 15, 1965,` and now Patent 3,268,411 issued August 23, 1966, andy entitled Closure Method and Means for the Ends ofy Nuclear Fuel Rods Having a Very Thin Sheath.

As can be gathered from FIGURE 3, in the case of large-diameter Ipencils where the central member 12 cant act as a support for contiguous pencils of the stack, in stead of the wires 14 rigidly connecting the member 12? to the cans, the pencil is borne by the spider 23 which isy rigidly connected to the member 12 and to the rings 7 and'. through which the coolant can ow and which mainly' comprises arms 24 bearing against the surface of the channel 17 to center and support the end zones of the pencil where the corrugations 10 are disposed.

The invention has been described with reference to particular em'bodiments but is not of course limited theretol and can be varied. For instance, the cans 1 and 2 can be: formed with longitudinal corrugations or folds permitting; radial distortion which are interleaved with corrugations- 9 strengthened by wires 13 and which are either of the: same height or of less height than the latter corrugations 9. Also, in the case of large-diameter Ipencils the ends thereof can be centered not by arms 24 of the spider 231 but by equidistant wings or the like on the ring 21.

We claim:

1. A tubular fuel pencil for a nuclear reactor compris-- ing tubular inner and outer sheath, said sheath defining; therebetween an annular space, a substantially cylindrical'A body of nuclear fuel, said nuclear fuel -being placed in said'V annular space, means for closing the ends of the said an nular space, inwardly projecting longitudinal corrugations'l on said inner sheath and a rigid cylindrical member' mounted within said inner sheath, said rigid cylindrical' member being rigidly secured to said end closing means,` said corrugations bearing against said cylindrical member to thereby support said inner sheath, and outwardly projecting longitudinal corrugations on said outer sheath.

2. A tubular fuel pencil as claimed in claim 1 in which longitudinal rods are fixed within said corrugations thereby strengthening said sheaths.

3. A tubular fuel pencil for a pressure fluid cooled power reactor, comprising a cylindrical outer sheath, a cylindrical inner sheath, said sheaths being assembled to define an annular space therebetween, a substantially hollow cylindrical member of ssile material, two annular plugs for closing the end portions of said sheaths, said sheaths being constructed of a exible material preferably of stainless steel no greater than .1 mm. in thickness,

longitudinal folds formed in said sheaths between portions receiving the annular plugs, said longitudinal folds permitting radial deformation of said sheaths, peripheral folds at least in part of the portion between the plug receiving portions permitting axial deformation of said sheaths, said sheaths havin-g equidistant longitudinal folds whose fold height is greater than the fold height f at least the folds permitting axial deformation, internally fixed supporting wires strengthening said equidistant longitudinal folds, a viscous layer of material which is plastic at the working temperatures of the pencil and the reactor coating said annular space, a rigid cylindrical member mounted in the interior of said inner sheath against whose surfaces said equidistant longitudinal Wire strengthened folds bear and the ends of said cylindrical member being rigidly secured to the pencil ends.

4. A tubular fuel pencil according to claim 3 wherein the cylindrical member is rigidly secured to the ends of the pencil by wires engaging in the ends of the rigid cylindrical member and to the closure plugs.

5. A tubular pencil according to claim 3 wherein the cylindrical member is rigidly connected to the pencil ends by means of disk-like members engaged with the ends of the cylindrical member and with the closure plugs.

6. A tubular fuel pencil according to claim S in which the cylindrical member is connected to the pencil ends by means of a spider means having equidistant outwardly projecting radial arms.

7. A tubular fuel pencil according to claim 3 in which additional longitudinal folds are provided [between the longitudinal folds, said additional longitudinal folds being parallel to the longitudinal folds and being of a reduced fold height.

8. A tubular fuel pencil according to claim 3 wherein said folds for permitting axial deformation of said sheaths are helical folds.

9. A tubular fuel pencil according to claim 3 wherein the peripheral folds are peripheral corrugations.

10. A tubular fuel pencil according to claim 3 in which the peripheral folds are helical corrugations formed as a continuation of the longitudinal folds permitting radial deformation of said sheath.

11. A tubular fuel pencil according to claim 3 in which the surface of the sheath between the longitudinal folds permitting radial deformation and the equidistant internally supported longitudinal folds contains corrugations parallel to the aformentioned folds but of lesser height.

References Cited by the Examiner FOREIGN PATENTS 978,737 12/1964 Great Britain.

CARL D. QUARFORTH, Primary Examiner.

BENJAMIN R. PADGETT, Examiner.

M. I. SCOLNICK, Assistant Examiner. 

1. A TUBULAR FUEL PENCIL FOR A NUCLEAR REACTOR COMPRISING TUBULAR INNER AND OUTER SHEATH, SAID SHEATH DEFINING THEREBETWEEN AN ANNULAR SPACE, A SUBSTANTIALLY CYLINDRICAL BODY OF NUCLEAR FUEL, SAID NUCLEAR FUEL BEING PLACED IN SAID ANNULAR SPACE, MEANS FOR CLOSING THE ENDS OF THE SAID ANNULAR SPACE, INWARDLY PROJECTING LONGITUDINAL CORRUGATIONS ON SAID INNER SHEATH AND A RIGID CYLINDRICAL MEMBER MOUNTED WITH SAID INNER SHEATH, SAID RIGID CYLINDRICAL MEMBER BEING RIGIDLY SECURED TO SAID END CLOSING MEANS, SAID CORRUGATIONS BEARING AGAINST SAID CYLINDRICAL MEMBER TO THEREBY SUPPORT SAID INNER SHEATH, AND OUTWARDLY PROJECTING LONGITUDINAL CORRUGATIONS ON SAID OUTER SHEATH.
 3. A TUBULAR FUEL PENCIL FOR A PRESSURE FLUID COOLED POWER RECTOR, COMPRISING A CYLINDRICAL OUTER SHEATH, A CYLINDRICAL INNER SHEATH, SAID SHEATHS BEING ASSEMBLED TO DEFINE AN ANNULAR SPACE THEREBETWEEN, A SUBSTANTIALLY HOLLOW CYLINDRICAL MEMBER OF FISSILE MATERIAL, TWO ANNULAR PLUGS FOR CLOSING THE END PORTIONS OF SAID SHEATHS, SAID SHEATHS BEING CONSTRUCTED OF A FLEXIBLE MATERIAL PREFERABLY OF STAINLESS STEEL NO GREATER THAN .1 MM. IN THICKNESS, LONGITUDINAL FOLDS FORMED IN SAID SHEATHS BETWEEN PORTIONS RECEIVING THE ANNULAR PLUGS, SAID LONGITUDINAL FOLDS PERMITTING RADICAL DEFORMATION OF SAID SHEATHS, PERIPHERAL FOLDS AT LEAST IN PART OF THE PORTIONS BETWEEN THE PLUG RECEIVING PORTIONS PERMITTING AXIAL DEFORMATION OF SAID SHEATHS, SAID SHEATHS HAVING EQUIDISTANT LONGITUDINAL FOLDS WHOSE FOLD HEIGHT IS GREATER THAN THE FOLD HEIGHT OF AT LEAST THE FOLDS PERMITTING AXIAL DEFORMATION, INTERNALLY FIXED SUPPORTING WIRES STRENGTHENING SAID EQUIDISTANT LONGITUDINAL FOLDS, A VISCOUS LAYER OF MATERIAL WHICH IS PLASTIC AT THE WORKING TEMPERATURES OF THE PENCIL AND THE REACTOR COATING SAID ANNULAR SPACE, A RIGID CYLINDRICAL MEMBER MOUNTED IN THE INTERIOR OF SAID INNER SHEATH AGAINST WHOSE SURFACES SAID EQUIDISTANT LONGITUDINAL WIRE STRENGTHENED FOLDS BEAR AND THE ENDS OF SAID CYLINDRICAL MEMBER BEING RIGIDLY SECURED TO THE PENCIL ENDS. 